1. Field of the Invention
The present invention relates to an absolute encoder, and more particularly to a transformation means for transformation into absolute position from an absolute pattern.
2. Related Background Art
Examples of conventional absolute encoders for transformation into absolute position from an absolute pattern, for example, of maximum-length linear feedback shift register sequence (M-sequence) are the absolute encoders disclosed in Japanese Patent Application Laid-open Nos. 57-175211 and No. 63-231215.
The absolute encoder disclosed in Japanese Patent Application Laid-open No. 57-175211 has a ROM as means for transformation into absolute position, with which each absolute signal detected is directly transformable into an absolute position corresponding to a position of the detection. In other words, all detection positions representing respective absolute positions are made transformable into absolute position, using the ROM, which is a look-up table for transformation from detected data into one of the absolute positions.
On the other hand, the absolute encoder as disclosed in Japanese Patent Application Laid-open No. 63-231215 is so arranged that a reference M-code generating circuit is first cleared, then to output different M-codes one by one at every input of one clock pulse until the output reaches an M-code (m) corresponding to a rotational position (.theta.) of a code plate.
When the reference M-code generating circuit outputs the M-code (m) corresponding to the rotational position (.theta.), a counter counts the number of clock pulses supplied before the output of the M-code (m), whereby it can be determined that the M-code (m) should correspond to the rotational position (.theta.).
As described above, the conventional absolute encoders performed the transformation into absolute position, using the ROM, or the pattern generating circuit such as the reference M-code generating circuit in the above prior art application.
In case of the former using the ROM for transforming all detection positions into the absolute positions, the necessary ROM capacity becomes larger as the pulse number (the number of minimum read units) of the absolute encoder increases. For example, a change from 2.sup.n pulses to 2.sup.n+1 pulses makes the capacity of the ROM 2.multidot.(n+1)/n times larger. If circuits of the absolute encoder are constructed of a semicustom IC such as a gate array, most gates of the semicustom IC are used up for the ROM portion for transformation into absolute position while few of them can be used for the control unit of the encoder. Thus, a problem was that the semicustom IC could not be used with good efficiency.
Further, in case of the latter using the pattern generating circuit for transformation into absolute position, the time necessary for transformation into absolute position has great variations depending upon a positional relation between the reference position and the detection position. For example, in case of an absolute encoder of 2048 pulses with clock 1 MHz, the necessary time is 1 .mu.s at minimum and 2.048 ms at maximum. Thus, there was a problem that if the absolute encoder was used to control a motor or the like, the difference in the necessary time was too large to execute a stable control.